Various filter systems have been developed to filter water and wastewater. Typical filter systems include but are not limited to an upflow filter, a downflow filter, a combined upflow filter and a downflow filter and multiple downflow filters connected in series. The term upflow filter is given to a filter in which the liquid or influent to be filtered is directed in an upward path to remove impurities. Conversely, a downflow filter is a filter in which the influent is directed in a downward path to remove impurities. In a combined upflow/downflow filter system, influent is initially directed upwardly through the upflow filter to remove a predetermined percentage of the impurities in the influent and then directed downwardly through the downflow filter to remove the remaining impurities to within an acceptable limit. Where the filter system includes multiple downflow filters connected in series, the influent is directed downwardly through each of the downflow filters one after the other to remove the impurities to within an acceptable limit.
Regardless of the type of filter system and/or the type of the liquid to be filtered, it is common for the filter system to include an underdrain. The underdrain directs and/or receives fluids during operation of various cycles of the filter system including the filtration cycle and the washing cycle. The fluids may include influent, pre-filtered washing water, effluent and air. A key function of the underdrain is to evenly distribute the washing fluid through the filter bed during a washing cycle so that at least a significant amount of the impurities trapped in the filter bed can be removed. In the event that the underdrain does not evenly distribute the washing fluid through the filter bed, the filter bed most likely will not be properly cleaned. At a minimum this will cause much shorter filtration cycles and more washing cycles. This is undesirable, as the filter system typically cannot operate in the filtration cycle during a washing cycle. Accordingly, the amount of filtered water produced by a filter system for a given period of time can be significantly reduced.
One type of underdrain system that is used in a filter is one or more porous plates anchored to one or more underdrain blocks. The porous plate is typically located adjacent the filter bed. The porous plate was developed at least in part to obviate the need for one or more gravel support layers between the filter media and the underdrain. The gravel support layers have been used to prevent the filter media from becoming lodged in the underdrain. However, the gravel support layers have several disadvantages including the fact that the gravel layers take up space in the filter that could be occupied by filter media. As such, gravel support layers inherently limit the amount of impurities that can be removed from a fluid during any given filtration cycle. However, underdrain systems using porous plates are not without problems. For example, the fine filter media can become lodged in various pores of the porous plate thereby preventing the washing fluid from passing through that portion of the porous plate. Accordingly, the washing fluid is not directed evenly through the filter bed inherently leading to shorter filtration cycles. This clogging problem can become so extensive over the porous plate that the pressure of the fluid immediately upstream of the porous plate becomes dangerously high causing the porous plate to lift off the underdrain necessitating the filter system to be shutdown for days or even months in extreme cases while this underdrain failure is corrected.
Other problems are present in underdrains apart from the clogging problem described above. For example, underdrains are designed to operate within estimated hydraulic and/or structural capacities. Filter systems can be inadvertently operated at high flow rates where the pressure exerted on the underdrain by the fluid passing upwardly there through exceeds design capacity. This could lead to the underdrain lifting or otherwise failing.
A preferred form of the present invention provides a method and apparatus for monitoring an underdrain of a filter system for filtering water or wastewater during operation of the filter that overcomes the disadvantages described above.